Various efforts have hitherto been made to increase the functionality of polymeric compounds. For example, in one approach currently used to increase the refractive index of polymeric compounds, aromatic rings, halogen atoms or sulfur atoms are introduced onto the compounds. Of such compounds, episulfide polymeric compounds and thiourethane polymeric compounds, both of which have sulfur atoms introduced thereon, are in practical use today as high-refractive index lenses for eyeglasses.
The most effective way to achieve even higher refractive indices in polymeric compounds is known to involve the use of inorganic metal oxides.
For example, a method for increasing the refractive index by using a hybrid material composed of a siloxane polymer mixed with a material containing small dispersed particles of zirconia, titania or the like has been disclosed (Patent Document 1).
A method in which a condensed ring skeleton having a high refractive index is introduced onto portions of a siloxane polymer has also been disclosed (Patent Document 2).
However, in materials that use inorganic metal oxides, there is a trade-off between refractive index and transparency, which makes it difficult to increase the transparency while retaining a high refractive index.
Accordingly, as a way to maintain transparency, investigations have been conducted on dispersing, within a film-forming composition, fine particles of an inorganic oxide having an average primary particle size no larger than the wavelength of visible light. A higher refractive index and a greater transparency have been achieved using this approach (Patent Document 3). The method used for dispersing fine particles of inorganic oxide is generally a technique that involves mixing together a polymer solution and a dispersion of inorganic fine particles, with the polymer playing the role of a binder which stabilizes, without destroying, the dispersion of inorganic fine particles.
In addition, numerous attempts have been made to impart heat resistance to polymeric compounds. Specifically, it is well known that the heat resistance of polymeric compounds can be improved by introducing aromatic rings. For example, polyarylene copolymers with substituted arylene recurring units on the backbone have been disclosed (Patent Document 4). Such polymeric compounds show promise primarily in use as heat-resistant plastics.
Melamine resins are familiar as triazine resins, but have very low decomposition temperatures compared with heat-resistant materials such as graphite.
The heat-resistant organic materials composed of carbon and nitrogen that have been in use up until now are for the most part aromatic polyimides and aromatic polyamides. However, because these materials have linear structures, their heat-resistance temperatures are not all that high.
Triazine-based condensation materials have also been reported as nitrogen-containing polymeric materials having heat resistance (Patent Document 6).
In recent years, there has arisen a need for high-performance polymeric materials in the development of electronic devices such as liquid-crystal displays, organic electroluminescence (EL) displays, optical semiconductor devices (LEDs), solid-state image sensors, organic thin-film solar cells, dye-sensitized solar cells and organic thin-film transistors.
The specific properties desired in such polymeric materials include (1) heat resistance, (2) transparency, (3) high refractive index, (4) high solubility, (5) low volume shrinkage, and (6) resistance to high temperatures and high humidity.
The inventors earlier discovered that hyperbranched polymers containing recurring units with a triazine ring and an aromatic ring have a high refractive index, are capable of achieving, with the polymer alone, high heat resistance, high transparency, high refractive index, high solubility and low volume shrinkage, and are thus suitable as film-forming compositions in the manufacture of electronic devices (Patent Document 6). However, when employing cured films produced from such compositions as light-extracting layers in organic EL devices or light-emitting diodes, a higher light-extracting efficiency (light-diffusing efficiency) is desired.
Although cured films produced from such compositions, when employed in electronic devices, are required to have durability under harsh conditions of high temperature and high humidity, this point has not hitherto been investigated.
It has been reported that films obtained from film-forming compositions prepared by dispersing inorganic fine particles in a binder resin composition having a refractive index of about 1.5 exhibit a resistance to high temperatures and high humidity (Patent Documents 7 and 8).
The inventors have already reported compositions obtained by the addition of inorganic fine particles to a triazine ring-containing hyperbranched polymer (Patent Document 9), although there are no reported examples of compositions that form films which exhibit resistance to high temperatures and high humidity or of light-scattering film-forming compositions.